CLIC RF manipulation for positron at CLIC Scenarios studies on hybrid source Freddy Poirier 12/08/2010

Efficiency snapshot ACS only Up to Target Global e-e- e+e+ Compton 1.3 GeVCompton 3 GeVHybrid Source 5 GeV Up to TargetACS only Results here with old aperture structures On the way to be updated Efficiency (here): Nb e+ /nb e- at 200 MeV, E>165 MeV Compton source HB source polarisation No polarisation  Sources scenarios (pol. and non- pol. ) being studied: - Hybrid source (CLIC baseline) - Compton source (see talks from I.Chaikoska) Single 5 ip As of 4 weeks ago

Hybrid Source Scenario (Baseline) 5 GeV 2 m between crystal and target 10 mm target thickness Positron Bunch length at target exit= 500  m e - /  Target (crystal) Pre-injector Linac for e + Primary beam Linac for e - 2 GHz  e  Target AMD 200 MeV5 GeV e-e- HB source Focus of the present study

Present Studies Capture and acceleration using Travelling Wave (TW) tanks. –84 accelerating cells tanks + 2 couplers i.e. long cavity –Prior studies have used Standing Wave (SW) 6 cells cavities (see talk of A.Vivoli) For Hybrid source (HB) –Scenarios with modification phase of first TW –Space charge limits –Additional Scenarios Solenoid field (1T) Cavity within AMD

2 GHz TW tanks 2 GHz 84 accelerating cells constitute the TW tanks –Note: 84 cells + 2 half cells for couplers within ASTRA –2  /3 operating mode 4.36 m long 15 MV/m Up to 5 tanks are used to accelerate e+ up to 200 MeV Typical cells dimension for the TW tanks Case with 4 cells 15 mm Typical electric field for the 4 cells cavity 2  /3 HB source First optimisation done on 15 mm iris (radius aperture) tanks but final results with 20 mm iris tanks

Capture Strategy Acceleration: Phase of the first tank tuned for use of maximum accelerating gradient for the first tank –4 tanks are needed to reach ~200 MeV Deceleration: adapt the phase and gradient of the first tank to capture a maximum of positrons –5 tanks are needed to reach ~200 MeV HB source e+e+  /e + target AMD Solenoid TW tanks Capture strategy of the first tank? Other tanks: full acceleration

Acceleration Acceleration case at m (to reach ~200MeV)  2388 positrons With cut at e min =165 MeV Total Efficiency (200MeV,emin) 2388/6000= E (MeV) Z (m) Population X (m) Transverse distribution HB source

Deceleration First tank Phase = 280 o Choice of gradient: 7Mv/m 6Mv/m 5Mv/m E (MeV) Z (m) More positrons in the higher tail but tail further in z More positrons in lower tail but tail lower in energy Gradient (MV/m) Nb of positrons > 165 MeV Nb of positrons > 185 MeV Eff= Eff= Values at m (at ~200 MeV) HB source

At 200 MeV For the deceleration case the efficiency* at 200 MeV is higher than for the acceleration case For further information: The value in red gives the number of positrons within the red dashed box Acceleration caseDeceleration case Eff 200MeV = 0.40Eff 200MeV = e e+ Z (m) Energy (MeV) *Eff 200 MeV : Nb of particles entering target/ Nb of particles at exit of tank with energy greater than 165 MeV HB source

20 mm aperture New 2 GHz cavity and field provided by P.Lepercq HB source Total yield=0.9 Efficiency >165MeV=4621/6000= 0.77 (was 0.4 for 15 mm aperture) Acceleration scenario Total yield=0.95 Efficiency >165MeV=5335/6000= 0.89 (was 0.53 for 15 mm aperture) Deceleration scenario This is the big advantage of the deceleration technique

Impact of space charge No space charge -10% level Expected level of charge for HB Charge x 250 In order to have a 10% level impact on the result, the e- charge has to be multiplied by 250. We are far from this with the present CLIC values. Negligible space charge effect to be expected x10 9 e HB: Acceleration scenario At 200 MeV HB source Charge per macro-particles (nC): C mp =Ne + i x qe + / ( 1e-9 x Ne + s Ne + i : initial nb of e + Ne + s : simulated nb of e +

Impact of space charge Acceleration scenario: –The use of space charge imply -3.6% e+ at 200 MeV. –With the present e - charge, we are at ~ lower than a 10% effect Deceleration scenario: –The impact is -4.6%. –With the present e - charge, we are at 10 2 lower than a 10% effect The impact of space charge is here of the order of less than 5% The difference from acceleration to deceleration is noticeable but the level is not detrimental in both case HB source

Additional Studies on the capture with hybrid source Additional scenario: –1) An accelerating field within the Adiabatic Matching Device (AMD) –2) Increase of the magnetic field surrounding the TW Accelerating section. Traditionally it is fixed at 0.5 T. e-e- e+e+  /e + target AMD 4 cells cavity Solenoid TW tanks 

Additional Scenarios Case (AMD 6T  0.5 T and no cavity inside the AMD, but AMD inner radius=15 mm) Acceleration scenarios e+e+  /e + target AMD 4 cells cavity Solenoid TW tanks HB source 20 mm nominal case

Conclusion 2 scenarios studied: –Acceleration and deceleration –Very interesting scenario is deceleration Further optimisation (on lattice) for capture can be done No major impact from space charge foreseen in both scenarios Additional scenarios are also studied (booster within the AMD and higher solenoid field) which shows good potentials Perspectives What about power consumption (TW/SW)? –The choice of TW/SW is not done yet. It will highly depends on the power consumption. –Though if TW are in use for acceleration between 200 MeV up to DR then it is an attractive choice. –Review of dimension should be done (2 GHz  1 GHz?) The results need to be sent to the bunch compressor and check the effective yield there HB source

More Slides

Compton Scenario l =1064 um 1.3 GeV e- 2o2o  ze- =600  m x 5 e-e- e+e+ target  z = 600  m  zl >  ze- Laser Pulse length=1.5 mm Laser pulse energy=0.1 J Compton source

Distributions at ~200 MeV Acceleration case (18.12 m)Deceleration case (22.63 m) 2138 e+1255 e+ Red box:  e=10 MeV,  z=5 mm 1.3 GeV, 5 IP This value is used for the efficiency calculation Compton source

Impact of space charge The space charge is considered traditionally low because: –Large transverse size of the beam at exit of AMD –High enough energy (The positrons have an average energy > 5 MeV so quite relativistic) –Previous studies with Parmela simulation have shown an impact of space charge of the order of 1 % on the result Are we far from any effect due to Space charge? –What is the limit wrt to the charge of the bunch. i.e. if we increase the charge of the bunch at the exit of the AMD will there be a noticeable impact? –What happen to the space charge limit if we do deceleration? HB source

The CLIC CDR yield hypothesis PDR PDR: Pre-Damping Ring e e - Yield e + /e - = 0.7 Target e+/e- Pre-accelerator We are simulating 6000 e - (macro-particles), representing e - i.e. the charge per simulated particle is nC. CDR values HB source